1. Field of Invention
This invention relates to short wavelength gas discharge lasers. In particular to pulsed extreme ultraviolet and Xray lasers.
2. Description of Prior Art
Pulsed Xray and extreme ultraviolet lasers operate on transitions between highly ionized atomic states. For example in Argon the fourth to fifteenth ionized states of the atom produces excited states that generate transitions in the soft Xray and extreme ultraviolet region of the electromagnetic spectrum. Many extreme ultraviolet lines with wavelengths below 200 nm have been observed, several have produced pulsed laser output. The conditions to produce Xray and extreme ultraviolet laser output are created during a pulsed gas discharge or in it's afterglow. Pulses with peak current densities of thousands of amps per square centimeter are required to generate large numbers of such highly excited states. To produce high peak currents a very low inductance electrical pulser configuration is required. Additionally the discharge is confined to narrow diameter cylindrical region by the laser geometry. The narrow bore produces a high current density. High peak current discharges are generated by the rapid switching of a capacitor. The capacitor is charged to a high voltage, typically 10-30 kilovolts. The energy stored in the capacitor is switched into the discharge by a fast high voltage switch such as a thyratron.
Heretofore described pulsed discharge Xray and extreme ultraviolet lasers have used continuous bore tubes. In these lasers a gas discharge is produced in a narrow bore tube. The tube may be filled with a low pressure gas. In some cases the inner wall material of the tube is evaporated during the electrical pulse to produce the active plasma of the discharge. A major disadvantage of this method is that the tube material is rapidly consumed by the discharge due to sputtering and evaporation even when the tube is filled with low pressure gas. Xray lasers have used narrow diameter continuous bores described as capillaries. The discharge is known as a capillary discharge. Similarly gas discharge extreme ultraviolet lasers have used larger diameter continuous bores constructed from fused silica.
The production of Xray laser output from a pulsed discharge requires high peak current densities in excess of 5000 amps per square cm. High current gas discharges like this are known to cause ion and electron sputtering and ion burial at surfaces near the gas discharge. Sputtering and ion burial results in gas loss from the laser system. The magnitude of the gas loss depends upon the discharge bore geometry, the discharge current and the construction material of the internal laser components. Fused silica has a relatively high sputtering and ion burial rate. Gas is lost rapidly in lasers constructed with fused silica bores and material from the bore contaminates the active laser gas. The contamination may absorb at the laser wavelength and reduce or extinguish laser output.
Additionally in these devices the pulsed electrical discharge produces a gas pumping effect in which the gas is pumped from one end of the tube to the other. The magnitude and direction of this pumping are dependent upon the discharge current and the pulse repetition rate and other factors. The gas pumping effect results in a reduction of laser output energy in sealed off systems. Often gas return lines are added to the laser to allow for pressure equalization. Also the laser may be operated non sealed off with a flowing gas source. A further problem caused by gas pumping is the differential pumping rates of different gases, this leads to gas separation in discharges consisting of binary or ternary gas mixtures.
The pulse repetition rate in continuous bore lasers is limited due to bore heating. The pulsed gas discharge causes heating which can melt or soften the bore at high repetition rates. This has limited the repetition rate of continuous bore lasers.
There is no sealed off long life laser source of extreme ultraviolet or Xrays.